The invention relates to a heat-exchange fluid and to its use.
Heat-exchange fluids are intended to cool numerous devices subjected to temperatures which are considered to be too high for the satisfactory operation of the device.
For example, they are used for the cooling of micro-processors, onboard electronics, or heat or electric engines.
They are also used in the cooling of nuclear reactors.
Water is one of the best fluids known as heat-exchange fluid.
However, additives, such as ethylene glycol or propylene glycol, may be added to it, which additives prevent it from freezing at excessively low temperatures.
However, wherever savings in weight are expected, the fact of being able to use a smaller amount of heat-exchange fluid (water, water+ethylene glycol) for identical, indeed even superior, heat-exchange properties is of great advantage.
Thus, it is necessary to increase the thermal conductivity of the heat-exchange fluid. It has been recently demonstrated that the addition of nanoparticles to a heat-exchange fluid significantly increases the thermal conductivity of the heat-exchange fluid. These novel heat-exchange fluids take the name of nanofluids (Choi (S.)—Enhancing Thermal Conductivity of Fluids with Nanoparticles.—The American Society of Mechanical Engineers, New York, Vol. 231/MD—Vol. 66: 99-105, November 1995, or Yu (W.), France (D.), Routbort (J.) and Choi (S.), Review and Comparison of Nanofluid Thermal Conductivity and Heat Transfer Enhancements.—Heat Transfer Engineering, Vol. 29, pp. 432-460 (2008), or Das (S.), Choi (S.), Yu (W.) and Pradeep (T.)—Nanofluids: Science and Technology.—J. Wiley (2008)).
The addition of various types of nanoparticles to a fluid in order to enhance the thermal properties thereof has been widely studied and it currently appears that it is not necessary for the nanoparticles used to be, by nature, composed of a good heat-conducting material, such as a metal, and that respectable performances can be obtained with materials which are thermally markedly less effective, such as clays or oxides: halloysite, laponite, silica (SiO2), zinc oxide (ZnO) or alumina (Al2O3), these materials being products which are available industrially. Parameters such as the robustness, that is to say the stability over time of the nanofluid in use, and the overall energy balance, that is to say the compromise between the increase in the thermal conductivity with respect to the increase in the viscosity of the fluid, can be evaluated.
By way of comparison, in laminar flow, the overall energy balance is considered to be positive when the increase in viscosity is less than 5 times the increase in the thermal conductivity.
This is because an excessively great increase in viscosity results in the need to increase the power of the pumping unit, which has the effect of nullifying all, or a large part, of the benefit obtained by the increase in thermal conductivity.
More particularly, alumina γ-Al2O3 and α-Al2O3 and its hydrated derivatives (Al(OH)3, AlOOH), as regards their industrial availability, their very low toxicity and the possibility of obtaining nanoparticles having numerous shapes, has been more particularly studied and its thermal conductivity is good for a material of oxide type (40 m−1K−1, for α-Al2O3).
For the hydrated forms of alumina, this thermal conductivity is much lower.
The influence of the shape and size of the alumina particles on the improvement in the thermal conductivity of water has been studied in particular by Timofeeva et al. in “Particle shape effects on thermophysical properties of alumina nanofluids”, Journal of Applied Physics, 106, 014304 (2009).
The conclusions of this paper are that alumina particles in the platelet form result in the lowest increase in the thermal conductivity of the heat-exchange fluid in which the alumina is incorporated, in comparison with alumina in the strip, slab or cylindrical form, and that, furthermore, the alumina in the platelet form is the alumina which brings about the greatest increase in the viscosity of the heat-exchange fluid in which it is incorporated, still in comparison with chemically equivalent aluminas having the strip, slab or cylindrical form.
Thus, among the alumina nanoparticles which can be used as additive in a heat-exchange fluid, those having a platelet form, that is to say whose smallest dimension is the thickness, are those which are the least suitable as, at an equivalent percentage by weight in the heat-exchange fluid, it is these which exhibit the weakest ability to increase the thermal conductivity of the fluid but which, on the other hand, greatly increase the viscosity thereof.